The present invention relates to an apparatus for enabling the use of an a-c tachometer output as the speed signal from a bi-directional d-c machine. More particularly, the present invention relates to a means for providing a d-c signal output which has an absolute value proportional to the speed of a bi-directional d-c machine and a polarity indicative of the direction of rotation of the bi-directional d-c machine and which utilizes an a-c tachometer output as the input speed signal.
In the past, the most common means of determining the speed of a motor was to use a d-c tachometer connected to the shaft of the d-c machine. The d-c tachometer is typically a permanent magnet generator which provides an output which is proportional to the speed of rotation and the polarity of which is a function of the direction of rotation. However, the d-c tachometer has several disadvantages. One of these disadvantages is that the output voltage for a predetermined speed of rotation is slightly different between the forward and reverse directions of rotation. Another disadvantage is that the d-c tachometer requies brushes which are subject to wear and which require periodic maintenance. An even more serious disadvantage is that explosion proof designs are difficult to achieve and expensive due to the need to protect against the possibility of arcing at the comutator's surface. These problems are not present when an a-c tachometer is used as the speed signal source. The a-c tachometer provides an output a-c voltage which is proportional to the speed of rotation. However, when using an a-c tachometer alone, the direction of rotation of the machine is not known.
The present invention overcomes these disadvantages by enabling the use of an a-c tachometer as the speed signal from a bidirectional machine by providing a means of converting the a-c output of the tachometer to a d-c signal having an absolute value proportional to the speed of the machine and a polarity which is a function of the direction of rotation of the machine.